1. Field of the Invention
This invention relates to an imaging apparatus for endoscopes which includes an endoscope and an imaging section having the function of a TV camera and allows an image derived from the endoscope to be observed through a TV monitor.
2. Description of Related Art
In general, where an image inside the human body which is derived from an endoscope is displayed on a TV monitor for observation, an imaging apparatus is used which has a TV camera removably mounted to the rear of an eyepiece section of the endoscope.
Imaging apparatuses for endoscopes of this type are various in optical arrangement. As an example, an imaging apparatus for endoscopes disclosed in Japanese Patent Preliminary Publication No. Hei 7-234365 may be cited. FIG. 1 shows the arrangement of this imaging apparatus. The imaging apparatus includes an adapter 13 removably mounted to the eyepiece section of an endoscope 11, having a focus lens 12 for changing the degree of convergence or divergence of a beam of light emerging from the eyepiece section, and a TV camera 17 of watertight structure which is removably mounted to the adapter 13 and has a variable stop 14, an imaging lens system 15, and an image sensor 16. In this way, the imaging apparatus is designed so that when the TV camera 17 is attached through the adapter 13 to the eyepiece section of the endoscope 11, a distance between the adapter 13 and the TV camera 17 is changed and thereby a focusing operation is performed.
FIGS. 2A and 2B show arrangements of conventional imaging apparatuses for endoscopes. Each of these imaging apparatuses is equipped with an endoscope 21 and a TV camera head 25 which has an imaging lens system 22 for re-forming an object image relayed, as a final image, by the endoscope 21, stop means 23 variable in aperture size, and an image sensor 24. The relative positions of at least one of lenses constituting the imaging lens system 22 and the stop means 23 are kept constant with respect to the optical axis. In FIG. 2A, the imaging lens system 22 and the variable stop 23 are moved along the optical axis, while in FIG. 2B, the image sensor 24 is moved. In either case, an axial distance between the one lens of the imaging lens system 22 and the image sensor 24 is changed and thereby the focusing operation is performed.
FIG. 3 depicts the arrangement of a TV photographic adapter constituting a conventional imaging apparatus for endoscopes. A TV photographic adapter 31 includes an imaging lens system 32, a variable stop 33, a power supply 34 for actuating the variable stop 33, a light-receiving element 35 for controlling the variable stop 33, and a variable stop control device 36. This adapter is hermetically sealed through sapphire glass covers 37. Although the TV photographic adapter 31, when used, is connected, together with a TV camera, not shown, to the endoscope, the focusing operation is performed by means of the same mechanism as in FIG. 1.
In the imaging apparatus for endoscopes, it is desirable that the stop is placed close to the exit pupil of the endoscope. This is because if the stop is disposed farther away from the exit pupil, a light beam passing through the stop will give rise to vignetting, and the quality of a resultant image will be deteriorated.
With the imaging apparatus shown in FIG. 1, when the focusing operation is performed, the position of the stop in the imaging apparatus for endoscopes is varied, and hence a chief ray passing through the lens system is shifted from the position where the optimum condition is obtained, that is, aberrations are most favorably corrected. Consequently, an image may suffer deterioration in quality, which is not favorable. Furthermore, the imaging apparatus provides the adapter 13 with a focusing function and thus requires a large number of parts. This fact is disadvantageous to costs.
The imaging apparatus shown in FIG. 2A, which is constructed so that the stop means 23 is moved, together with the imaging lens system 22, along the optical axis, is unfavorable for use because the relative positions of the stop means 23 and the exit pupil of the endoscope 21 are shifted to thereby cause variations of aberrations as in the imaging apparatus of FIG. 1. In order to drive the stop means 23 directly, the imaging apparatus requires a multiple frame structure (at least two frames, one for fixing the stop means 23 and the imaging lens system 22 and the other for the focusing operation), with a resulting increase in manufacturing cost. Moreover, since the stop means 23 is moved, together with a unit holding the stop means 23, along the optical axis, there is the fear that a signal line, not shown, is broken which is connected to the stop means 23 to transmit a signal for controlling the aperture size thereof.
On the other hand, the imaging apparatus shown in FIG. 2B, which is such that the image sensor 24 is moved along the optical axis in the focusing operation, requires a length of fit or a unit including the imaging lens system 22 to be increased to some extent so that optical misalignment of the entire lens system is obviated. This brings about an unfavorable shape for the imaging apparatus for endoscopes requiring lightweight and compact design. Additionally, by moving the image sensor 34, an unnecessary load is applied to a flexible printed circuit board provided at the rear end thereof, and hence there is the possibility that the flexible printed circuit board will be damaged.
The TV photographic adapter 31 of the imaging apparatus for endoscopes shown in FIG. 3 is equipped with the power supply 34 for actuating the variable stop 33 and the control device 36, which form the key elements that prevent lightweight and compact design required for the imaging apparatus for endoscopes. Thus, this arrangement is unfavorable. The focusing means of this apparatus has the same problem as that of the apparatus shown in FIG. 1.